1. Field of the Invention
The present invention relates to an apparatus for auto focusing, and more particularly, to an apparatus for auto focusing that can remove noise caused by an actuator driving signal when sensing a position of a lens.
2. Description of the Related Art
With the development in techniques for digital cameras, auto focusing that automatically adjusts focus of a camera has attracted much attention. In particular, the camera module has emerged as an essential function for a mobile terminal like a cellular phone, and thus the importance of associated additional functions has been increased. In order to implement the currently commercialized auto-focusing, displacement of a lens needs to be applied. To this end, an actuator like a motor is being used. Representative examples of the actuator may include a voce coil actuator (VCA) and a piezoelectric actuator. As the cellular phone is reduced in size and consumes less power, the camera module and an auto-focusing module also need to have low power consumption and small size.
In general, the auto-focusing module may include a sensing unit that senses a position of a lens and an actuator that changes the sensed position of the lens. The piezoelectric actuator has been widely used in the camera module since it has low power consumption and small size. However, the piezoelectric actuator has a high peak current while driving, which may act as noise in the sensing unit that senses the position of the lens.
FIGS. 1A and 1B are graphs illustrating noise occurring in the sensing unit by a square wave signal of an actuator in an apparatus for auto-focusing according to the related art.
In FIG. 1B, a driving signal for driving a piezoelectric actuator is shown. The driving signal may be a square wave signal having a rising edge {circle around (1)}′ and a falling edge {circle around (2)}′. In FIG. 1A, a sensing voltage in the sensing unit is shown. As shown in FIG. 1A, noise components {circle around (1)} and {circle around (2)} occur in the sensing voltage at the edges of the actuator driving signal.